Change Nexus Player HDMI output resolution

I have a UWHD 2560 x 1080 Display, and the Nexus Player would only output 1920 x 1080 resolution, leaving 2 massive black pillars on each side. I know that Android could theoretically support any screen resolution, but the stock rom does not have any settings to change the HDMI output resolution. I have tested sideloading a play store app called "Resolution Changer Pro" but it did not work.

Does anyone know how to do this? Or is there any custom rom that could do this?

Thanks in advance!